Organic resin materials are used for applications in various fields by taking advantage of characteristics thereof, such as impact resistance, light weight, and processability. Efforts are currently made to take more advantage of these properties. One such approach is to apply molded organic resins having enhanced surface hardness and abrasion resistance to the windows in various vehicles. In the glazing application, a high level of abrasion resistance and outdoor weather resistance comparable to glass are required. In the case of automobiles, for example, a high level of abrasion resistance is required in order to prevent the windshield from scratching upon wiper operation and to prevent side windows from scratching upon winding up-and-down operation. Potential service in a very high temperature and/or humidity environment must also be taken into account.
Conventionally, there are known coating compositions for forming a surface protecting coat film intended to provide high hardness and scratch resistance on the surface of an organic resin substrate made of plastic or the like. The coating compositions are made of a composition obtained by hydrolyzing or partially hydrolyzing a hydrolysable organosilane or are prepared by mixing colloidal silica into the composition.
For example, Patent Document 1 to 3 disclose coating compositions that contain organoalkoxysilane, a hydrolysate of the organoalkoxysilane and/or a partial condensate thereof, and colloidal silica, and in which the alkoxy group is converted with excess of water into silanol. However, there is a problem that these coating films obtained by the wet coatings were insufficient in hardness and inferior to glass as a target to be substituted in terms of scratch resistance.
In order to solve the problem of the insufficient scratch resistance in the coating films obtained by the wet coatings, there is also disclosed a method for providing an outermost layer of an additional hard film obtained by plasma polymerizing an organosilicon compound on the wet coating layer. It has been known that using the method enables obtaining of a laminate having scratch resistance equal to or higher than glass.
However, in order to form a coating film that can withstand sunlight and wind and rain for a long time, there is still another problem. The wet coating layer having scratch resistance described above cannot block ultraviolet light, thus causing a phenomenon in which the resin substrate, a primer layer for providing adhesion to the substrate, or an interface therebetween is deteriorated and/or discolored by ultraviolet light. Methods disclosed for preventing the phenomenon are to add an ultraviolet absorber to the primer layer and to introduce an ultraviolet-absorbing organic substituent into an organic resin forming the primer via a chemical bond. The terms “ultraviolet absorber” and “ultraviolet-absorbing organic substituent” as used herein refer to, for example, substituents such as benzophenone, benzotriazole, and triazine and organic compounds containing the substituents (see Patent Document 4 to 7).
The above-mentioned methods are those in which ultraviolet light is blocked by the primer layer containing the organic ultraviolet absorber. However, originally, the primer layer is mainly intended for improving adhesion between the underlying organic resin substrate and a silicone layer. Accordingly, when the amount of the ultraviolet absorber added is too much, problems occur, such as adhesion reduction and transparency reduction. Additionally, long-term outdoor exposure testing and accelerated weather resistance testing have shown that blocking of ultraviolet light by merely introducing the ultraviolet absorber and/or the ultraviolet-absorbing organic substituent into the primer layer is insufficient to prevent deterioration and discoloration of the organic resin substrate.
On the other hand, as a method for compensating for the disadvantages, an organic ultraviolet absorber has also conventionally been added to a silicone layer. However, simply adding such a compound to a coating composition does not improve durability of a coating film resulting from the composition. In other words, the ultraviolet absorber bleeds and flows out from the surface after long-term exposure, lacking persistence. Then, methods of using a silyl-modified organic ultraviolet absorber have also been disclosed to date that can form a chemical bond with a siloxane compound which is the main component of a coating layer (see Patent Document 8 to 1). In the methods, strong bonding of the ultraviolet absorber to the siloxane matrix has improved persistence, while the scratch resistance of the coating layer has been significantly reduced, or micro-cracks has been significantly occurred due to lack of flexibility thereof. Thus, the use of an organic ultraviolet absorber is inherently disadvantageous in that the hardness of a silicone film is reduced as the amount of the ultraviolet absorber to be added is increased in order to improve weather resistance.−+
In addition, such a wet coating system requires steps of laminating a plurality of layers, although a high level of weather resistance can be provided. Accordingly, simplification of the steps is urgently required in terms of reducing manufacturing time, increasing yield, and lowering ultimate cost.
The structure of an actual wet coating system is composed of at least three layers: an organic resin substrate, a primer layer, and a wet coating layer. The primer layer as used herein is mainly formed by an acrylic coating, and the wet coating layer is mainly formed by a silicone hard coating. In other words, the primary wet coating is applied and cured on the organic resin substrate, and then additionally, the silicone hard coating is applied and cured thereon to form a laminate.
When forming, as an outermost layer, a hard coating film obtained by plasma polymerizing an organosilicon compound in order to obtain scratch resistance equal to or higher than glass, a plasma polymerization step will be further added to the above-described steps.
Additionally, when forming a hard layer having scratch resistance equal to or higher than glass by plasma polymerizing an organosilicon compound, the condition for forming a dense silicon oxide layer causes a state in which the silicone hard coat layer is pulled by the silicon oxide layer. Then, the hard coat layer cannot follow the silicon oxide layer, whereby cracking defects occur in the hard coat layer during weather resistance testing.
To overcome such a disadvantage and to simplify the steps, a technique is disclosed for forming a hard film by directly plasma polymerizing an organosilicon compound on a primer wet coating layer or a photocurable (meth)acrylic hard coat layer (see Patent Document 12). However, it is known that an acrylic resin used in the primer wet coating layer or the photocurable hard coat layer is decomposed in a chain-reaction manner in a plasma (polymerization) environment. Thus, there has been a problem in that performing plasma polymerization in a strong plasma environment to form a dense silicon oxide layer causes decomposition of the underlying layer to reduce weather resistance.
As described above, the various attempts have been made to improve weather resistance, scratch resistance, etc., of wet and dry coating films. However, there has been no method for manufacturing a laminate whose coating film exhibits visible light transparency and ultraviolet light shielding properties while maintaining an extremely high level of scratch resistance (e.g., comparable to that of glass), and further fully satisfies weather resistance and durability enough to withstand long-term outdoor exposure.